The present invention relates to a pressure sensor which undergoes physical movement in response to an applied external force. This invention also relates to techniques for fabricating such a pressure sensor.
Conventional silicon micromachined pressure sensors typically use either piezo-resistive or capacitive elements to sense the deflection of a thin silicon diaphragm. Piezo-resistive elements are much more common than capacitive elements because the piezo-resistive elements have a lower cost, as well as greater product familiarity and acceptance.
FIG. 1A is a top view of a conventional silicon micromachined piezo-resistive pressure sensor 1. Pressure sensor 1 is fabricated on a silicon substrate 2 having an area of 2 mm by 2 mm and a thickness on the order of 500 xcexcm. To increase the sensitivity of pressure sensor 1, substrate 2 is fabricated to include a frame 2a, an annular diaphragm 2b and a circular platform 2c. Diaphragm 2b is etched to have a thickness on the order of 10 xcexcm, while frame 2a, and platform 2c remain at a thickness of approximately 500 xcexcm. As a result, the deformation of substrate 2 will be concentrated within the annular diaphragm 2b, thereby increasing the sensitivity of pressure sensor 1.
Four Wheatstone bridge circuits 3a, 3b, 3c and 3d are formed on substrate 2. Each of these Wheatstone bridge circuits includes a plurality of contact pads 4, a plurality of piezo-resistive elements 5, and conductive traces for connecting the pads 4 and piezo-resistive elements 5. Piezo-resistive elements 5 are formed by ion implanting impurity regions into the annular diaphragm 2b. The resistances of piezo-resistive elements 5 change in response to mechanical stresses applied to the crystalline substrate 2. More specifically, the resistances of piezo-resistive elements 5 change in response to compression and dilation of diaphragm 2b. This annular diaphragm 2b and the position of piezo-resistive elements 5 provides a 25 to 50 times increase in the gauge factor, such that pressure sensor 1 can provide an output voltage on the order or 2 to 3 mV/V when designed for full range of differential pressure on the order of a 4 inch water column (WC).
In the past, pressure sensor 1 has typically been used for high pressure range sensing applications in the automobile world. Such applications include, for example, measurements of manifold absolute pressure, transmission fluid pressure, coolant and power steering pressure and tire pressure.
The effectiveness of pressure sensor 1 is determined by a combination of two physical effects, which can be explained in terms of a mechanical amplifier cascaded with an electrical amplifier. The mechanical amplifier is diaphragm 2b which converts pressure into displacement. The electrical amplifier is the combination of piezo-resistive elements 5 and Wheatstone bridge circuits 3a-3d, which convert displacement into output voltage.
There are a number of inherent disadvantages associated with pressure sensor 1. First, platform 2c acts as a seismic mass which causes an excessive amount of dynamic deflection in response to shock and vibration (i.e., noise). Platform 2c can further cause an excessive amount of static deflection in response to gravity, thereby making the sensor highly sensitive to mounting positions). As a result, the operation of pressure sensor 1 can be affected by the position and environment in which pressure sensor 1 is mounted.
In addition, piezo-resistive elements 5 act as pyro-resistors, thereby making pressure sensor 1 extremely sensitive to temperature changes. As a result, sophisticated temperature compensation schemes must typically be used with pressure sensor 1. It is typical that even after such temperature compensation is provided, the temperature effects are on the order of 1 to 2 percent of full range.
Furthermore, annular diaphragm 2b is typically very fragile, thereby rendering pressure sensor 1 prone to damage during transportation, handling and assembly. Also, while the annular diaphragm 2b increases the sensitivity of the mechanical amplifier portion of pressure sensor 1, the shape of annular diaphragm 2b limits the linear elastic range the diaphragm 2b. As a result, the performance of pressure sensor 1 can be nonlinear if the deformation of diaphragm 2b exceeds the linear elastic range of the silicon diaphragm.
Moreover, because of the inherent stiffness of silicon substrate 2, pressure sensor 1 is better suited for high pressure applications (i.e., measuring pressures greater than 1 psi), rather than low pressure applications (i.e., measuring pressures less than 1 psi).
FIG. 1B is a cross sectional view of a conventional capacitive differential pressure sensor 20 which is used to measure pressure. Pressure sensor 20 is formed by sandwiching an etched silicon diaphragm 29 (which is etched from a silicon substrate 28) between an upper glass plate 30 and a lower glass plate 27. Pressure ports 25 and 26 are formed through the upper and lower glass plates 30 and 27, respectively, to vent silicon diaphragm 29. Aluminum is sputtered to the inner surfaces of the upper and lower glass plates to form fixed capacitor plates 23 and 24. Connectors 21 and 22 extend from plates 23 and 24, respectively, along the walls of pressure ports 25 and 26, to the outer surfaces of the upper and lower glass plates 30 and 27. The silicon diaphragm 29 forms a movable center capacitive plate of the sensor 20 in a configuration similar to a capacitive potentiometer. A positive pressure applied to pressure port 25 causes the silicon diaphragm 29 to deflect toward the lower glass plate 27, thereby increasing the capacitance between diaphragm 29 and plate 24, while decreasing the capacitance between diaphragm 29 and plate 23. The imbalance, which is directly proportional to pressure, is detected by an electronic circuit.
Pressure sensor 20 to exhibits the following disadvantages. First, silicon diaphragm 29, being relatively thick (i.e., having a thickness of at least about 5 microns), can experience an excessive amount of dynamic deflection in response to shock and vibration. Furthermore, as silicon diaphragm 29 is made thinner for low pressure applications (i.e., a thickness of approximately 5 microns) it is difficult to fabricate a substantially planar diaphragm. A non-planar diaphragm can result in erroneous capacitance measurements. Moreover, as silicon diaphragm 29 is made thinner for low pressure applications, the diaphragm becomes very fragile, thereby rendering pressure sensor 20 prone to damage during transportation, handling and assembly.
It would therefore be desirable to have a low-cost, reliable pressure sensor which is relatively insensitive to temperature, dynamic shock and gravitational forces. It would also be desirable if such pressure sensor is relatively sturdy and has a wide linear elastic range. It would further be desirable if such pressure sensor were well suited for low pressure applications.
Accordingly, the present invention provides a sensitive pressure sensor which includes a flexible membrane, such as low-stress silicon nitride, which is supported by a semiconductor frame. The flexible membrane extends over the frame, and an inherent tensile stress is present in the membrane. A thin film strain gage material, such as nickel-chrome, is deposited over the flexible membrane to form one or more variable resistance resistors over the flexible membrane.
When an external pressure, such as a dynamic pressure drop due to an air flow, is applied to the membrane, the membrane is deformed out of plane. When the membrane is deformed out of plane, the variable resistance resistors increase in length, and thereby increase in resistance. The increase in resistance is monitored by an electronic circuit, such as a Wheatstone bridge circuit. The sensor circuit generates an output signal which is proportional to the deflection of the membrane. Since there is only tensile stress in a membrane (as opposed to both tensile and compressive stresses in a diaphragm which is thicker and can support bending), the output signal provided by the pressure sensor of the invention is the same whether the membrane is deformed up or down, such that the output signal is proportional to the differential pressure.
Because there is no proof mass attached to the flexible membrane, the pressure sensor of the present invention is immune to shock, vibration, and orientation. In addition, the resistance of the strain gage material is based purely on geometric effect, such that the pressure sensor of the present invention is very temperature stable. Furthermore, the flexible membrane undergoes a relatively large deformation (in comparison with sensor 1) for a given applied pressure differential. This provides a relatively sensitive and stable sensor, suitable for low pressure applications.
The flexible membrane is made of a strong material which can withstand a large applied pressure differential. Because the membrane is under tensile stress, the out-of-plane displacement is linearly proportional to the applied pressure. Moreover, because the membrane is thin, its mass is negligible, such that the applied gravitational forces resulting from mishandling are much too small to damage the pressure sensor.
The pressure sensor of the present invention can be mounted as a single die in a standard housing. Alternatively, the pressure sensor can be mounted between two silicon dies, which act as over-pressure stoppers to limit the deformation of the membrane.
In accordance with another embodiment, a first conductive layer is formed over the membrane, and a second conductive layer is formed over an over-pressure stopper. In this embodiment, the first and second conductive layers to form a capacitive pressure sensor.